The present invention generally relates to testing, measuring, analyzing or predicting bioaccumulation. More particularly, the present invention relates to methods for determining the partition coefficient of chemical substances, wherein the partition coefficient may relate to the bioaccumulation of such substances. The methods of the present invention may be particularly suitable for measuring or evaluating bioaccumulation of surfactants, although the method may also be used for measuring or evaluating bioaccumulation of other chemical substances.
Bioaccumulation is generally defined as the process through which a chemical increases in concentration in a biological organism over time when compared to the concentration of the chemical in the environment. Compounds accumulate in living things any time they are taken up and stored faster than they are broken down, metabolized or excreted. The process is normal and can be helpful to life, as in the storage of vitamins, for example. However, the process can result in injury to life when the equilibrium between exposure and bioaccumulation is overwhelmed. The extent of bioaccumulation depends on the concentration of the chemical in the environment, the amount of chemical coming into an organism from the food, air or water, and the time it takes for the organism to acquire the chemical and then store, metabolize or degrade, and excrete it. The nature of the chemical itself, such as its solubility in water and fat, affects its uptake and storage; the ability of the organism to degrade and excrete the chemical also affects its uptake and storage. Understanding the dynamic process of bioaccumulation is generally viewed as important in protecting humans and other organisms from adverse effects from chemical exposure. Consequently, bioaccumulation has become a critical consideration in the regulation of chemicals.
Industries using chemicals in the environment are increasingly faced with regulations concerning bioaccumulation of those chemicals. The oil and gas industry has varying guidelines and regulations in many countries worldwide relating to chemicals used in the search for and production of hydrocarbons from subterranean formations in those countries. Some regulations require testing of individual components of chemicals used. For compliance with such guidelines and regulations, the industry tests its chemicals and chemical components, often by test methods or techniques also prescribed, recommended, and/or approved in the guidelines or regulations.
Guidelines and regulations pertaining to bioaccumulation frequently refer to a value known as a substance's partition coefficient. The partition coefficient, often represented as “P” or written in the form of its logarithm to base ten, “log P,” is the ratio of the equilibrium concentrations of a dissolved substance in a two-phase system consisting of two largely immiscible solvents. For example, the partition coefficient of a test substance in solvents n-octanol and water may be written as Pow, and calculated as the quotient of the equilibrium concentration of the test substance in n-octanol (Cn-octanol) and the equilibrium concentration of the test substance in water (Cwater), expressed as follows:       P    ow    =            C              n        ⁢                  -                ⁢        octanol                    C      water      
The partition coefficient for octanol and water solvents, Pow, is a key parameter in studies of the environmental impact of chemical substances. The Organisation for Economic Co-operation and Development's (“OECD”) Guideline for Testing of Chemicals No. 117 states that there is a highly significant relationship between the Pow of substances and their bioaccumulation in fish and that Pow is useful in predicting adsorption on soil and sediments and in establishing quantitative structure-activity relationships for a wide range of biological effects.
One test that has been used to determine the partition coefficient for n-octanol and water is the High Performance Liquid Chromatography (HPLC) Method described in the OECD Guideline for Testing of Chemicals No. 117, incorporated herein in its entirety by reference and available from OECD in Paris, France. This test is performed on analytical columns packed with a commercially available solid phase containing long hydrocarbon chains (e.g., C8-C18) chemically bound onto silica. Chemicals injected onto such a column move along it by partitioning between the mobile solvent phase and the hydrocarbon stationary phase. The chemicals are retained in proportion to their hydrocarbon-water partition coefficient, with water-soluble chemicals eluted first and oil-soluble chemicals eluted last. This pattern enables the relationship between the retention time on a reverse-phase column and the n-octanol/water partition coefficient to be established. The partition coefficient is deduced from the capacity factor, k, given by the formula:   k  =                    t        R            -              t        o                    t      o      where tR is the retention time of the test substance, and to is the dead-time, i.e., the average time an unretained molecule needs to pass through the column. Quantitative analytical methods are not needed and only the retention times are measured.
In general, before the Pow value is determined through a test such as the HPLC Method, a preliminary estimate of Pow is made using known calculations. This preliminary estimate may then be used to select which test will be used to measure the Pow value more precisely, as certain tests may only be able to reliably determine Pow values within a limited range. For example, the HPLC Method is useful in determining Pow values when log Pow is in the range between 0 and 7. When the log Pow value is estimated to be in the range between −2 and 4, another test has been used. That test is the OECD Guideline for Testing of Chemicals No. 107, called the Partition Coefficient (n-octanol/water): Shake-Flask Method, which is incorporated herein in its entirety by reference and available from the OECD in Paris, France.
The Shake-Flask Method is based on the principle that the Nernst partition law applies at constant temperature, pressure and pH for dilute solutions. OECD Guideline No. 107 states that the law strictly applies to a pure substance dispersed between two pure solvents and when the concentration of the solute in either phase is not more than 0.01 mole per liter. If several different solutes occur in one or both phases at the same time, the results may be affected. Dissociation or association of the dissolved molecules cause deviations from the partition law.
Neither the HPLC Method nor the Shake-Flask Method is suitable for calculating the partition coefficients for chemicals that are considered surface active, or surfactants. Nevertheless, surfactants are commonly used in drilling and well treating fluids. It has been previously disclosed that a slow-stirring or no-stirring method may be used to determine the partition coefficients of various surfactants. According to those methods, once a quantity of surfactant has been allowed to equilibrate between two immiscible solvents, the equilibrium concentration of the surfactant in each solvent may be measured and the partition coefficient may be calculated. One problem that may arise during the performance of these methods is the formation of surfactant micelles. It is well understood that when molecules (or ions) of a test substance such as a surfactant are mixed with a solvent in a concentration above the critical micelle concentration (“CMC”), the molecules (or ions) may associate to form micelles. The term “micelle” is defined to include any structure that minimizes the contact between the lyophobic (“solvent-repelling”) portion of a test substance molecule and the solvent, for example, by aggregating the test substance molecules into structures such as spheres, cylinders, or sheets, wherein the lyophobic portions are on the interior of the aggregate structure and the lyophilic (“solvent-attracting”) portions are on the exterior of the structure. Because the presence of micelles in a solvent may distort the readings of the equilibrium concentration of the test substance in the solvent, prior art methods that do not take steps to ensure that micelles will not form during equilibration may not yield accurate measurements of the equilibrium concentration. Micelles may also be problematic because, inter alia, micelles may be related to the problematic emulsification of the substances being tested.
The present invention generally relates to testing, measuring, analyzing or predicting bioaccumulation. More particularly, the present invention relates to methods for determining the partition coefficient of chemical substances, wherein the partion coefficient may relate to the bioaccumulation of such substances. The methods of the present invention may be particularly suitable for measuring or evaluating bioaccumulation of surfactants, although the method may also be used for measuring or evaluating bioaccumulation of other chemical substances.
In some embodiments, the present invention provides a method comprising: providing a test substance, providing two solvents that are substantially immiscible, introducing a known amount of the test substance and known amounts of the two solvents into a single vessel to create a pre-equilibrium sample, adjusting the concentration of the test substance in the pre-equilibrium sample so that the concentration of the test substance is below the critical micelle concentration of both solvents if the concentration of the test substance is not already below the critical micelle concentration, allowing the test substance to equilibrate between the two solvents over time at a substantially constant temperature, determining the equilibrium concentration of the test substance in each of the solvents, and calculating the partition coefficient.
The features and advantages of the present invention will be readily apparent to those skilled in the art. While numerous changes may be made by those skilled in the art, such changes are within the spirit of the invention.